I'm used to GNOME Classic and personally don't want to change to anything else, but it seems there is no more GNOME Classic in the Oneiric release. After apt-get installed gnome-session and all, though there comes back the GNOME Classic option in the session listbox, however, after logged in, it seems not the expected one.

There is no System menu in the top bar, and I couldn't customize panels at all. I want to change the appearance(theme), but I can't find Appearance menu at all. And much more differences to my previous experience of GNOME classic shell.

There are multiple valid answers for this question spanning over several versions of Ubuntu. For your convenience an index of each below:

Fixing the look of the panel in 11.10

As far as just the look of the panel goes, see
my answer
to
How to customize the gnome classic panel
for how to get back quite close to the original look. This is accomplished by, amongst other things, reducing the top panel's height and its icon sizes.

The first official release will be 13.04, which will be available by the end of April 2013. If you're itching to try it now, check out the daily images here.

Ubuntu GNOME (formerly Ubuntu GNOME Remix) is a mostly pure GNOME desktop experience built from the Ubuntu repositories. As the 12.10 release was our first release, there are still a few minor issues, but we proudly recommend Ubuntu GNOME to anyone interested in using Ubuntu with the best of what GNOME has to offer.

You can install this the same as any other Ubuntu from a live DVD and forgo anything related to Unity. Or install it as a dual boot alongside Ubuntu.

Can you add instructions how people can install the desktop on their existing computers? This makes it sound like you need to reinstall a new ISO to use GNOME Classic.
–
Jorge CastroApr 22 '13 at 13:46

1

It does not sound like it... it is a new ISO install ;)
–
RinzwindApr 22 '13 at 13:47

2

Ok so what about people with upgrades and who don't want to reinstall? Does the 12.04 and newer answers still work?
–
Jorge CastroApr 22 '13 at 13:59

There is another option. A small group of folks are working on a fork of Gnome 2 called Mate (pronounced MAH-tay, I think; could be wrong on the accent, but I'm reasonably certain it's meant to be two syllables). Its future is still uncertain, but the Linux Mint folks liked it enough to make it an option in their newest release. If you're willing to work outside the Ubuntu repositories, it's pretty easy to install. From the wiki:

I tested it on Oneiric 32-bit, and it installs fine. Takes 310MB of storage, according to apt-get. Didn't require any packages to be removed or updated, so it should play well with others. Note that it's still a young project, though, so bugs and quirks are likely.

There's no Ubuntu branding/theming by default, but it takes the built-in themes (like Ambiance) reasonably well. And it includes the much-missed (for me, anyway) System menu!

I haven't been able to find any differences between Gnome 2 and MATE. It seems to be a simple name change that enables you to run both Gnome 3 and Gnome 2 at the same time. That's why one man is able to do this as a hobby, whereas Gnome is based on the work of thousands of developers. The extreme hype this project has received, does not reflect reality. I would rather recommend using Gnome Panel in Gnome 3. Good answer, though. +1 :)
–
Jo-Erlend SchinstadDec 29 '11 at 20:16

Umm... I think GNOME 3 should rename to something else, though. Because there are so many changes. GNity?
–
Xiè JìléiJun 2 '12 at 2:48

It depends on what you want instead. You can no longer use Gnome2, at least not as easily. You can do a sudo apt-get install gnome-session for Gnome3 (which is worse than Unity imo) or you can get the lightweight xfce4 desktop sudo apt-get install xfce4, which is older looking, but very similar in functionality to Gnome2 (possibly even better). Another popular alternative is KDE, which you would get with sudo apt-get install kubuntu-desktop. Removing unity should be possible with sudo apt-get purge unity, though be careful, as it could inadvertantly uninstall software you may want. I recommend against removing Unity completely, and instead using one of the above desktop managers along side it.

To switch which one you use, just hit the little gear icon above your password field on the login screen.

Click this link to install the Gnome Flashback package, find it in the Ubuntu Software Center, or type sudo apt-get install gnome-session-flashback in a terminal. There are more instructions here

If you are running 12.04-13.10 fellow this instructions.

Click this link to install the Gnome Fallback/Panel package, find it in the Ubuntu Software Center, or type sudo apt-get install gnome-panel or sudo apt-get install gnome-session-fallback in a terminal. There are more instructions here

You need to get the Ambiance theme if you don't ready have it. It isn't any different from the stock one that you need. If you don't have it click here to download it And just install it though Gdebi, Ubuntu Software Center, or tools like it. Finally you just need to chance the wallpaper to the one in 10.04 and you do that by just opening a terminal and type in

First, Remove the the button panel by right clicking on it and select Delete. Add the Show Desktop, Window Buttons, Workspace Switcher and Trash Applet to the new button panel. Now we are done with the button panel now it is time for the button one so chance the panel size to 24px by right clicking on it and select Panel>Panel Preferences. Rename “File Manager” menu into “Places” by opening the handy terminal and type in sudo nano /usr/share/applications/xfce-settings-manager.desktop find the entry Name and change the entry into this Name=Settings Manager → Name=System save it and close it. Now Right click on the Panel Select Panel Preferences Than Items. Remove all default items and left 8 eight items : Applications Menu, Separator, Notification Area, add a separator between Applications Menu and Notification Area to be Expand by click the item, and click edit item by select Edit the currently selected item button at the bottom of Panel Preferences menu. Then, add two Launcher items and move them to the position after Applications Menu item. Set them to Show Label Instead of Icon, by clicking Edit the currently selected item>Advanced> Show Label Instead of Icon. Now, edit each launcher and add the two main applications. For first launcher, add the Places items by clicking Edit the currently selected item>General>Add new item, search and scroll down to get the Places item. For the second launcher, do the same but put the System item (that we have edited before from the xfce-settings-manager desktop menu).
Now, we have a good looking Gnome 2 panels.

You just need to download another desktop environment that you like the best and then switch to that environment on the login screen. I think it keeps your preference for the next time you get on, so you don't have to select your environment of choice every time.